The present invention relates to a liquid-crystal display, and in particular, to a color liquid-crystal display in which molecules of liquid crystal are aligned vertically (homeotropically) between two substrates when an electric field is not applied to the liquid crystal molecules and in which each pixel is divided into a plurality of domains.
FIGS. 10A to 10C are cross-sectional diagrams respectively showing black display, intermediate-tone display, and white display states of a prior-art liquid-crystal display of homeotropic alignment type in which liquid-crystal molecules are aligned homeotropically. A space or gap between a pair of substrates 100 and 101 is filled with liquid-crystal material 102 having a negative dielectric constant anisotropy. Polarizers are disposed respectively on outer surfaces of the substrates 100 and 101 such that polarization axes thereof are orthogonal to each other.
As shown in FIG. 10A, in a state in which a voltage is not applied to the liquid-crystal display, the liquid-crystal molecules 102 are aligned in a direction vertical or orthogonal to the substrates 100 and 101 and hence black display state is achieved. When a voltage is applied across the substrates 100 and 101, the liquid-crystal molecules 102 are aligned in a direction horizontal to the substrates 100 and 101 as shown in FIG. 10C. In this situation, a direction of polarization of light passing the liquid-crystal layer rotates to achieve white display state.
As shown in FIG. 10B, when a voltage lower than that applied for the white display state is applied across the substrates 100 and 101, the liquid-crystal molecules 102 are aligned in an inclined direction with respect to the substrates 100 and 101. A display state of an intermediate tone or color can be obtained by light L1 propagating in a direction vertical to the substrates 100 and 101. For light L2 propagating from a lower-right corner to an upper-left corner in FIG. 10B, an effect of birefringence of the liquid-crystal layer rarely occurs. Therefore, the screen is black when viewed in a direction from the upper-left corner to the lower-right corner. Conversely, for light L3 propagating from a lower-left corner to an upper-right corner in FIG. 10B, the effect of birefringence of the liquid-crystal layer considerably takes place. Therefore, the screen has a color like white when viewed in a direction from the upper-right corner to the lower-left corner. As above, in an ordinary liquid-crystal display of homeotropic type, the visual angle characteristic is lowered in the intermediate-tone display state.
To improve the visual angle characteristic, a liquid-crystal display of multidomain type has been proposed in which each pixel is divided into a plurality of domains. In the display of this type, liquid-crystal molecules are aligned in one direction in each domain in the intermediate-tone display state, and the alignment direction of liquid-crystal molecules in a domain is different from that in the adjacent domains. Referring to FIGS. 11A and 11B, description will be given of an example of a structure and operation of a liquid-crystal display of multidomain and homeotropic type (multidomain, vertically alignment (MVA) type).
FIG. 11A shows a cross section of a liquid-crystal display in a no-voltage state in which a voltage is not applied thereto. A first protrusion pattern 16 is disposed on an inner surface of a glass substrate 1. A second protrusion pattern 18 is disposed on an inner surface of an opposing substrate 36 facing the glass substrate 1. The first and second protrusion patterns 16 and 18 are arranged alternately. On the opposing surfaces of the glass substrate 1 on which a thin-film transistor (TFT) is formed and the opposing substrate 36, vertical alignment films 28 are respectively formed to cover the protrusion patterns 16 and 18. A space between the glass substrate 1 and the opposing substrate 36 is filled with liquid-crystal material 29 including liquid-crystal molecules 30. The molecules 30 have a negative dielectric constant anisotropy. On outer surfaces of the glass substrate 1 and the opposing substrate 36, a polarizer 31 and a polarizer 32 are respectively disposed in a cross-Nicol layout.
In a voltage-applied state in which a voltage is applied to the display, the liquid-crystal molecules are aligned in a direction vertical to the surfaces of the substrates 1 and 36. On inclined surfaces or planes of the first and second protrusion patterns 16 and 18, the liquid-crystal molecules 30a tend to be aligned in a direction vertical to the inclined plane associated therewith. Consequently, these molecules 30a are aligned obliquely with respect to the substrate surfaces. However, since the molecules 30 are aligned in the vertical direction in a wide area of the pixel, black display state is achieved satisfactorily.
FIG. 11B shows a cross-sectional view in an intermediate-tone display state in which a voltage is applied thereto to incline the direction of the liquid-crystal molecules. The liquid-crystal molecules 30a are inclined greater in inclined directions. Liquid-crystal molecules 30 around the inclined molecules 30a are also inclined in the same directions under the influence of the inclination thereof. Therefore, liquid-crystal molecules 30 between the first and second protrusion patterns 16 and 18 are aligned such that a longitudinal axis (a director) of each molecule 30 is aligned in a direction to an upper-right corner of FIG. 11B. Liquid-crystal molecules 30 on the left side of the first protrusion pattern 16 and those on the right side of the left protrusion pattern 18 are aligned such that the director of each molecule 30 is aligned in a direction to a lower-right corner of FIG. 11B.
As above, there are defined in one pixel a plurality of domains. The direction of inclination of liquid-crystal molecules in the domains varies from each other. The first and second protrusion patterns 16 and 18 define boundaries between the domains. By disposing the first and second protrusion patterns 16 and 18 in parallel to the substrate surfaces, two kinds of domains can be defined. By bending each protrusion pattern by 90xc2x0, four kinds of domains are defined. When a plurality of domains are defined in each pixel, the visual angle characteristic can be improved in the intermediate-tone display state.
The MVA-type liquid-crystal display obtains white display state using the effect of birefringence of the liquid-crystal material as described for FIG. 10C. Since the effect of birefringence has wavelength dispersion, the transmittance varies among the red (R), green (G), and blue (B) pixels in the white display state. This leads to coloring of the display screen.
FIG. 12 shows a relationship between the transmittance and a cell gap for each of the red, green, and blue pixels. The abscissa represents the cell gap in micrometer (xcexcm) and the ordinates represents the transmittance in percent (%). The transmittance is an overall transmittance of the entire liquid-crystal panel including the polarizers. The red, green, and blue pixels are equal in an opening ratio to each other. When the cell gap is set to a value from 4 xcexcm to 4.5 xcexcm for which the transmittance of the green pixel takes a maximum value, the transmittance of the blue pixel is lower than those of the red and green pixels. Therefore, the overall display screen becomes yellowish in the white display state.
It is therefore an object of the present invention to provide a liquid-crystal display capable of minimizing the coloring of the display screen in the white display state.
According to one aspect of the present invention, there is provided a liquid-crystal display, comprising a first substrate and a second substrate disposed in parallel to said first substrate with a gap therebetween; liquid-crystal material filled in a space between said first and second substrates, said liquid-crystal material having a negative dielectric constant anisotropy; an alignment film for homeotropically aligning liquid-crystal molecules of said liquid-crystal material in a non-electric field state in which an electric field is not applied to said liquid-crystal material; pixel electrodes formed on an opposing surface of said first substrate, said pixel electrodes defining pixels regularly arranged in a direction of rows and in a direction of columns, each said pixel electrode having a slit therein; a common electrode formed on an opposing surface of said second substrate; a color filter disposed for each said pixel for giving a color of red, green, or blue thereto; and protrusion patterns disposed on said opposing surface of said second substrate, each said protrusion pattern being disposed to divide an area of said pixel into a plurality of sub-areas when viewed in a direction of a normal of said substrates. Said slit of said pixel electrode is disposed apart from said protrusion pattern by a gap when viewed in a direction of a normal of said substrates, said slit and said protrusion patterns divide an area of said pixel into a plurality of domains, and said slits belonging to at least one selected from three groups of red, green, and blue pixels have width different from those of said slits of other pixel groups.
The transmittance is changed by changing the slit width. Using the variation in the transmittance, the difference of transmittance by the wavelength dispersion can be compensated for. By reducing the difference in the transmittance for each color pixel, the coloring of the display screen can be minimized in the white display state.